Brightness enhancing films can be used in a variety of applications, for example, interior illumination, light guides, and liquid crystalline displays (LCDs) such as those found in computer monitors. When employed in LCDs, one or more brightness enhancing films are used to increase the amount of light directed towards the viewer. This allows lower intensity, and thus less costly, bulbs to be used in the LCD. A backlight illuminates the liquid crystal display panel to desirably provide a uniformly intense light distribution over the entire plane of the LCD display panel. A backlight system typically incorporates a light pipe to couple light energy from a light source to the LCD panel. An array of diffusing elements can be disposed along one surface of the light pipe to scatter incident light rays toward an output plane. The output plane directs the light rays into and through the LCD panel. The backlight can use a light modulating optical substrate with prismatic or textured structures to direct light along a viewing axis, usually normal to the display and to spread illumination over a viewer space. The brightness enhancement optical substrate and diffuser film combinations enhance the brightness of the light viewed by a user and reduce the display power required to produce a target illumination level. This increase in brightness is customarily reported as the “gain,” which is the ratio of luminance using the brightness-enhancement film to the luminance without using the brightness-enhancing film, both measured on-axis, that is, in a direction perpendicular to the plane of the film towards the viewer.
It is also known to place two sheets of light directing film adjacent one another with their prisms oriented approximately perpendicular to one another to further increase the amount of light directed approximately normal to the axis of the display. While this construction effectively increases the amount of on axis light exiting the display, the resulting structure can exhibit uneven light transmission across the surface area of the display under certain conditions. This uneven light transmission is typically manifested by visibly apparent bright spots, streaks, or lines on the surface of the display; a condition caused by optical coupling between contacting, or very nearly contacting, surfaces of the adjacent sheets of light directing film, also known as “wet-out”. Wet-out occurs as a result of optical coupling between the prisms of one sheet and the smooth surface of the other. The optical coupling prevents total internal reflection from occurring along these peaks. The result is a mottled and varying appearance to the backlight. Such visibly apparent variations in the intensity of transmitted light across the surface area of the display are undesirable.
Additionally, for brightness enhancing films in a display that is intended for close viewing, such as a computer display, the cosmetic requirements are very high. This is because, when such displays are studied very closely or used for an extended period of time, even very small defects can be visible and annoying. Elimination of such defects can be very costly both in inspection time and discarded materials.
A second type of film used in LCDs is a diffusion film. As the name suggests, the diffusion film diffuses light directed to the viewer in order to reduce interference patterns such as Moire patterns. Such diffusers will hide many of the defects, making them invisible to the user. This will significantly improve manufacturing yield, while only adding a small increase in cost to the manufactured part. The disadvantage of this approach is that the diffuser will scatter the light and thus decrease on-axis gain. Therefore, a diffuser will increase yield but at the expense of some performance.
Hence, there is a continuing need for optical film systems that retain gain while reducing visible defects and wet-out.